1. Field of the Disclosure
This disclosure teaches techniques related to a magnetism detection device and method using a fluxgate circuit, and more particular to a magnetism detection device and method using a three-axis fluxgate circuit.
2. Description of the Related Art
Geomagnetic sensors are devices used for measuring the geomagnetic intensity and orientation that human beings cannot feel. In particular, geomagnetic sensors using a fluxgate circuit are referred to as fluxgate sensors. Such geomagnetic sensors measure the geomagnetic field of an observation point and indicate the orientation of the geomagnetic field, so that they can be used for diverse purposes such as providing map information displayed on vehicle navigation devices, hand-held phones, portable terminals, and so on.
On the other hand, the fluxgate-type magnetism detection devices such as the fluxgate sensors use a high-magnetic permeability substance such as permalloy as a magnetic core. An excited magnetic field is applied from a drive coil to the magnetic core. Secondary harmonic components in proportion to an external magnetic field are measured by use of non-linear magnetic characteristics. Thus the intensity and orientation of the external magnetic field is measured.
Such fluxgate-type magnetism detection device was developed in the late 1930's. These devices have good sensitivity, low cost, and are relatively small size when compared to the other magnetism detection devices. Further, such fluxgate-type magnetism detection devices consume less power and have excellent long-term stability. Such devices are widely used for civilian and military purposes including ore vein prospecting, target detections, artificial satellite posture controls, and space probes. In addition, they are used for the detection of weak magnetic fields and measurements of the absolute orientation of the Earth. Significant research is ongoing to improve the performance.
FIG. 1 is a block diagram for showing a structure of a conventional fluxgate-type magnetism detection device. In FIG. 1, the conventional fluxgate-type magnetism detection device 100 comprises a drive pulse generation circuit 110, a coil-driving current amplification circuit 120, a two-axis fluxgate circuit 130, a chopping circuit 140, a primary amplification circuit 150, a low-pass filter 160, a secondary amplification circuit 170, an analog/digital converter 180, a controller 191, and a memory 193.
The drive pulse generation circuit 110 generates drive pulses for driving the two-axis fluxgate circuit 130, selectively outputs and applies the drive pulses to the coil-driving current amplification circuit 120. The coil-driving current amplification circuit 120 uses plural amplifiers and inverters to output pulse signals and inverted pulse signals whose phases are opposite to the pulse signals output from the drive pulse generation circuit 110.
The two-axis fluxgate circuit 130 is provided with X-axis and Y-axis fluxgate sensors that are perpendicular to each other, driven by the pulse signals and inverted pulse signals that are respectively sent to the X-axis and Y-axis fluxgate sensors, and outputs a detection signal corresponding to an electromotive force induced due to the drive signals. The X-axis and Y-axis fluxgate sensors have two rectangle-shaped magnetic cores respectively installed in length directions of the X and Y axes, and each of the magnetic cores has a drive coil and a detection coil wound thereon. If the drive pulses are applied to the drive coil, a magnetic field is generated around the X-axis and Y-axis fluxgate sensors, so that the fluxgate sensors can detect an induced electromotive force through the detection coil.
The electric signal detected by the two-axis fluxgate circuit 130 controls plural switches built in the chopping circuit 140 for chopping. The chopped electric signal is differential-amplified in the primary amplification circuit 150, filtered to include only the signal of a certain range using the low-pass filter 160, and finally amplified in the secondary amplification circuit 170. The amplified signal is converted to a digital voltage value in the A/D converter for an output.
The controller 191 uses the maximum and minimum values of the X and Y axes that are stored in the memory 193 such that a bias value of the X axis is an average value of the maximum and minimum values of the X axis and a scale value of the X axis is a value obtained from dividing a difference of the maximum and minimum values of the X axis by two.
A normalized value is obtained by subtracting the bias value of the X axis from a digital voltage value output from the X-axis fluxgate sensor and dividing a subtraction result by the scale value. A normalized value of the Y axis is obtained by repeating the same process for the X axis as above. On the other hand, FIG. 2 is a graph for showing normalized digital voltage values output from the X-axis and Y-axis fluxgate sensors. The normalized output values of the X-axis fluxgate sensor and the normalized output values of the Y-axis fluxgate sensor are expressed as functions of cos( ) and sin( ), respectively. That is, a reference number 201 denotes normalized output values of the X-axis fluxgate sensor, and a reference number 202 denotes normalized output values of the Y-axis fluxgate sensor.
Further, the controller 191 calculates an azimuth at a current state by use of the corrected digital voltage values so that a current azimuth can be generated.
The function of tan−1( ) is mainly used for a process for calculating azimuths by use of the corrected digital voltage values. That is, the controller 191 normalizes the measured fluxgate voltage values of the X and Y axes, and uses an ideal value cos ψ of the function of cos( ) and an ideal value sin ψ of the function of sin( ) in Equation 1 as below for the calculation of an azimuth.
                              ψ          =                                    tan                              -                1                                      ⁡                          (                                                                                          sin                      ⁢                                                                                          ⁢                      ψ                                                                                                                                  cos                      ⁢                                                                                          ⁢                      ψ                                                                                  )                                      ,                            [                  Equatio          ⁢                                          ⁢          n          ⁢                                          ⁢          1                ]            wherein ψ denotes an azimuth, and cos ψ and sin ψ denote corrected digital output values from the X-axis and Y-axis fluxgate sensors.
When the above process is used, an azimuth is obtained by taking a value of tan−1( ) in the first quadrant in which output values are all positive. 180° is added to an angle obtained from a value of tan−1( ) not only in the second quadrant in which the values of the X axis are negative and the values of the Y axis are positive but also in the third quadrant in which the values of the X and Y axes are all negative. 360° is added to an angle obtained from a value of tan−1( ) in the fourth quadrant in which the values of the X axis are positive and the values of the Y axis are negative.
However, if azimuth calculation is performed as above, the controller can be overloaded due to the complicated calculation procedures. The maximum and minimum values of the digital voltage values have to be stored in advance for calculations of amplitudes and the offset values necessary for the calibration procedures. This causes wastage of memory resources. Further, in case that the azimuth calculation process as above is employed, the linear and non-linear portions of the digital voltage values are all used, which brings about a drawback of generating errors at 0°, 90°, 180°, and 270°.